1. Field of the Invention
This invention relates to an electron-emitting device and, more particularly, it relates to an electron-emitting device having a stable emission current as well as to an electron source and an image-forming apparatus using such electron-emitting devices.
2. Related Background Art
There have been known two types of electron-emitting device: the thermionic type and the cold cathode type. Of these, the cold cathode type refers to devices including field emission type (hereinafter referred to as the FE type) devices, metal/insulation layer/metal type (hereinafter referred to as the MIM type) electron-emitting devices and surface conduction electron-emitting devices. Examples of FE type devices include those proposed by W. P. Dyke & W. W. Dolan, "Field Emission", Advances in Electron Physics, 8, 89 (1956) and C. A. Spindt, "Pysical Properties of Thin-Film Field Emission Cathodes with Molybdenum Cones", J. Appl. Phys., 47, 5248 (1976).
Examples of MIM devices are disclosed in papers including C. A. Mead, "Operation of Tunnel-Emission Devices", J. Appl. Phys., 32, 646 (1961).
Examples of surface conduction electron-emitting devices include one proposed by M. I. Elinson, Radio Eng. Electron Phys., 10, 1290 (1965).
A surface conduction electron-emitting device is realized by utilizing the phenomenon that electrons are emitted out of a small thin film formed on a substrate when an electric current is forced to flow in parallel with the film surface. While Elinson proposes the use of an SnO.sub.2 thin film for a device of this type, the use of an Au thin film is proposed in G. Dittmer, "Thin Solid Films", 9, 317 (1972) whereas the use of In.sub.2 O.sub.3 /SnO.sub.2 and of a carbon thin film are discussed respectively in M. Hartwell and C. G. Fonstad, "IEEE Trans. ED Conf.", 519 (1975) and H. Araki et al., "Vacuum", Vol. 26, No. 1, p. 22 (1983).
FIG. 23 of the accompanying drawings schematically illustrates a typical surface conduction electron-emitting device proposed by M. Hartwell. In FIG. 23, reference numeral 201 denotes a substrate. Reference numeral 202 denotes an electroconductive thin film normally prepared by producing an H-shaped thin metal oxide film by means of sputtering, part of which eventually becomes an electron-emitting region 203 when it is subjected to current conduction treatment referred to as "energization forming" as will be described hereinafter. In FIG. 23, the narrow film arranged between a pair of device electrodes has a length G of 0.5 to 1 mm and a width W' of 0.1 mm.
Conventionally, an electron emitting region 203 is produced in a surface conduction electron-emitting device by subjecting the electroconductive thin film 202 of the device to a preliminary treatment, which is referred to as "energization forming". In the energization forming process, a constant DC voltage or a slowly rising DC voltage that rises typically at a rate of 1V/min. is applied to given opposite ends of the electroconductive thin film 202 to partly destroy, deform or transform the film and produce an electron-emitting region 203 which is electrically highly resistive. Thus, the electron-emitting region 203 is part of the electroconductive thin film 202 that typically contains a fissure or fissures therein so that electrons may be emitted from the fissure. Note that, once subjected to an energization forming process, a surface conduction electron-emitting device comes to emit electrons from its electron emitting region 203 whenever an appropriate voltage is applied to the electroconductive thin film 202 to make an electric current run through the device.
Japanese Patent Application Laid-Open No. 6-141670 discloses another configuration of a surface conduction electron-emitting device. It comprises a pair of oppositely disposed device electrodes of an electroconductive material and a thin film of another electroconductive material arranged to connect the device electrodes. An electron-emitting region is produced in the electroconductive thin film when the latter is subjected to energization forming. FIGS. 2A and 2B schematically illustrate a typical known surface conduction electron-emitting device (although its configuration also applies to an electron-emitting device according to the invention which will be described hereinafter).
With such an electron-emitting device, the intensity of electron beam emitted from the device can be remarkably improved by subjecting it to a process referred to as "activation". For an activation process, the device is placed in a vacuum apparatus and a pulse voltage is applied between the device electrodes until carbon or a carbon compound is produced from a tiny amount of organic substances existing in the vacuum and deposited near the electron-emitting region to improve the electron-emitting performance of the device.
Such a device is advantageous over a device proposed by M. Hartwell because the electroconductive thin film including an electron-emitting region of the device of the above invention can be independently prepared so that a material that can be reproducibly subjected to energization forming such as electroconductive thin film composed of fine particles may be used for it. This feature provides a particularly preferable advantage when a large number of surface conduction electron-emitting devices that operate uniformly for electron emission have to be manufactured.
However, with the current technological status, the emission current Ie of a surface conduction electron-emitting device cannot be satisfactorily controlled so as not to show any inadmissible fluctuations. In other words, the intensity of electron beam emitted from a surface conduction electron-emitting device is incessantly fluctuating and, in a surface conduction electron-emitting device of the above-mentioned another configuration, the ratio of the average emission current &lt;Ie&gt; to the deviation .DELTA.Ie is about 10% after a stabilization process, which will be described hereinafter.
Obviously, the ratio has to be made as small as possible in order to finely control the intensity of electron beam emitted from a surface conduction electron-emitting device as such a finely controllable device will find a broader scope of application.
The electron-emitting performance of a surface conduction electron-emitting device can show a sort of memory effect that the performance is irreversibly changed depending on the highest voltage that has been applied to the device. Fluctuations in the emission current Ie can be accompanied by fluctuations in the effective voltage applied to the electron-emitting region of the device and hence the electron emitting performance of a surface conduction electron-emitting device can be changed when a high voltage is applied to it as a result of such fluctuations in the effective voltage and gradually degraded in the course of time if the application of such a high voltage is repeated.
Conceivable causes of such fluctuations in the emission current Ie that lead to a degraded electron-emitting performance include (1) changes in the work function due to adsorption and desorption of gas molecules remaining in the vacuum to the electron-emitting region, (2) deformation of the electron-emitting region due to ion bombardments and (3) diffusion and movements of atoms of the electron-emitting region.
Techniques for suppressing such fluctuations in the emission current Ie and consequent degradation of the electron-emitting performance of a surface conduction electron-emitting device that have been proposed to date include the use of an external resistor connected in series to the device. However, when it comes to an electron source prepared by arranging a large number of electron-emitting devices, the use of a single external resistor connected in series to it cannot sufficiently nor satisfactorily suppress fluctuations in the emission current Ie of each of the electron-emitting devices.
An improvement to this technique may consist in the use of a plurality of resistors respectively connected to the electron-emitting devices of the electron source. However, it is not feasible to equalize the resistances of a large number of resistors and the use of resistors with uneven resistances can boost the deviations that exist in the performance of individual electron-emitting devices. Additionally, once the resistors are connected to the electron-emitting devices, the former have to be subjected to an energization forming process with the latter to baffle any efforts for optimizing the energization forming.
In view of the above identified problems, therefore, there has been a demand for electron-emitting devices provided with respective appropriate resistors that can be formed after an energization forming operation as well as a method of manufacturing such devices.